1. Field of the Invention
The present invention relates to a vibration-type driving device having a vibration element and a driven element, and more particularly to the structural arrangement of the vibration element.
2. Description of Related Art
Various conventional ultrasonic actuators for linear motion include a flat-plate-like vibration element. U.S. Pat. No. 5,698,930, for example, discloses such an ultrasonic actuator, which has an elastic body formed as a flat-plate-like vibration element. Vibrations can occur in the flat-plate-like vibration element in both a longitudinal vibration (oscillation) mode and flexural vibration mode. Also, displacement motions caused by vibrations in these modes are combined with each other to generate an elliptical movement or circular movement.
Another element (member) known as the driven element is kept in contact with the plate-like vibration element to cause relative movement between both elements.
Another example of an ultrasonic actuator with a flat-plate-like vibration element is disclosed in Japanese Laid-Open Patent Application No. Hei 6-311765. Here, the flat-plate-like vibration element is excited in two flexural vibrations. The structural arrangement of that vibration element is described with reference to FIGS. 35A, 35B and 35C.
In FIG. 35A, there is shown a flat-plate-like elastic body 100. A piezoelectric element 101 is bonded to one surface of the elastic body 100. Vibrations occur in a first flexural vibration mode, the behavior of which is shown in FIG. 35B, and in a second (out-of-plane) flexural vibration mode, the behavior of which is shown in FIG. 35C.
The behavior of vibration shown in FIG. 35B indicates that the first flexural vibration mode has cross-like nodes X1 and Y1, in which vibratory displacement motions in the Z direction are caused at protruding portions 102a, 102b, 102c and 102d. The behavior of vibration shown in FIG. 35C indicates that the second, out-of-plane flexural vibration mode has two nodes X2 and X3 in the Y direction, in which vibratory displacement motions in the X direction are caused at the protruding portions 102a to 102d. 
In FIG. 35A, the driven element (an element to be driven), not shown, is disposed on contact portions 105a, 105b, 105c and 105d, on the tips of the protruding portions 102a to 102d. The driven element is kept in pressure contact with the contact portions 105a to 105d. 
Vibratory displacement motions caused by the first and second flexural vibrations are combined with each other, so that an elliptical movement can be generated within the X-Z plane on the tips of the protruding portions 102a to 102d. Accordingly, the vibration element and the driven element can be moved relative to each other.
However, there are disadvantages associated with the above-described conventional vibration-type driving devices. In the first type (U.S. Pat. No. 5,698,930), which excites flexural vibration and longitudinal vibration, vibrations are excited at the plate-like elastic body in a longitudinal-vibration primary mode, so that it is difficult to reduce the size of the plate-like vibration element because an increased resonance frequency can result. Correspondingly, the vibration amplitude of the elliptical movement cannot be enlarged. This can cause driving performance to fluctuate in individual driving devices.
In the second type, which excites two flexural vibrations, vibrations are excited at the plate-like elastic body in the second, out-of-plane flexural vibration mode of FIG. 35C, so that there is an advantage in reducing the size of the elastic body. However, in the first vibration mode of FIG. 35B, cross-like nodes occur in this flexural vibration mode, so that the distribution of strains becomes complicated. Accordingly, some measures are required to excite such vibrations.
Further, the protruding portions 102a to 102d are distant from positions that cause the maximum amount of displacement in the Z direction in the flexural vibration mode shown in FIG. 35B. Therefore, it is difficult to provide sufficient amount of vibratory displacement to the protruding portions 102a to 102d, and to exert a larger driving force on the driven element.